(1) Field of the Invention
The present invention relates to the field of nuclear medicine systems. Specifically, the present invention relates to signal processing systems for scintillation detectors.
(2) Prior Art
Gamma cameras performing Single Photon Emission Computed Tomography (SPECT) have been utilized in nuclear medicine for some time. Anger proposed and developed such a system in the 1950s which has been modified and improved extensively with the introduction of high speed digital computer systems for image acquisition as well as image reproduction. However, SPECT camera systems utilize a collimator that is installed in front of the scintillation crystal within a scintillation detector. The collimator is used to collimate the incoming gamma rays so that only rays of a certain narrow angle of incidence actually penetrate the crystal. Although SPECT imaging is extensively used in nuclear medicine and provides beneficial image quality, the collimator introduces a source of image degradation in nuclear medicine images and tends to somewhat reduce the resolution and quality of images acquired by SPECT systems.
Cameras performing Positron Emission Tomography (PET) have been utilized in nuclear medicine as well with the introduction of relatively high speed detection electronics and computer systems for image acquisition and processing. These PET camera systems utilize a form of the scintillation detector that is used in SPECT systems, however, they do not utilize a collimator. In PET systems, the detection of two gamma rays in coincidence (in different scintillation detectors) is used to compute imaging information. A PET system employing two scintillation detectors is described in a paper presented by Gerd Muehllehner, M. P. Buchin, and J. H. Dudek entitled "Performance Parameters of a Positron Imaging Camera," published in the IEEE Transactions on Nuclear Science, Volume NS-23, No. 1, on February 1976 and also in a paper entitled "Performance Parameters of a Longitudinal Tomographic Positron Imaging System" by Paans, deGraaf, Welleweerd, Vaalburg and Woldring, in Nuclear Instruments and Methods, Volume 192, Nos 2, 3, on Feb. 1, 1982 pages 491-500. By utilizing higher energy gamma rays and eliminating the collimators, PET systems offer greatly improved image resolution and image quality over SPECT systems. Because the collimators are removed in PET systems, the detected count rate is higher in PET cameras over SPECT camera systems. Although both camera systems utilize different detection electronics and other circuitry, both PET and SPECT systems employ scintillation detectors.
The detection hardware for SPECT and PET systems is different in terms of the manner in which the systems detect and record events and is also different because PET systems operate at higher count rates over SPECT systems. Further, SPECT systems employ a different radionuclide over PET systems and detect gamma rays at different energy levels over PET systems. For this reason, although SPECT and PET systems are versatile and useful within nuclear medicine, in the prior art, different camera systems have been implemented and supplied for PET and SPECT imaging. Therefore, a facility desiring to perform SPECT and PET imaging is required to acquire two separate camera systems at a relatively greater expense.
It would be advantageous to provide a nuclear camera system offering the ability to perform both SPECT and PET imaging techniques within a single configurable system. Therefore, the expense of acquiring two separate systems can be advantageously avoided. The present invention offers such advantageous capabilities.
Moreover, non-uniform photon attenuation is an important factor that affects the quantitative accuracy of images collected using PET and SPECT camera systems and can decrease the sensitivity of these systems for lesion detection. Non-uniform photon attenuation creates image degradation by interfering with and partially absorbing the radiation emitted from an organ containing a radio-pharmaceutical. Photon attenuation within PET and SPECT systems tends to degrade images by also introducing image artifacts and other distortions that can result in false positive detection of lesions or the failure to detect lesions. The effects of photon attenuation are especially complex in cardiac studies as a result of nonuniform attenuation attributed to the thorax.
Transmission computed tomography (TCT) can be used as a method for generating a nonuniform attenuation correction distribution. In transmission scanning, the source of radiation is directed toward the associated scintillation detector through the object of interest or patient. The transmission image data is gathered using a known source (e.g., line, sheet, or flood) of radiation. If performed separately from the SPECT emission or PET study, the collection of the transmission data requires additional data acquisition time and the collection of the transmission and emission data is susceptible to misregistration effects due to patient (e.g., "object") movement between the data gathering sessions. It would be advantageous to provide attenuation correction within a dual head camera system switchable between SPECT and PET modes. The present invention provides such advantageous capability.
The transmission study may be performed simultaneously with a SPECT emission study. Among other advantages, simultaneous acquisition of both transmission and emission data (in SPECT) reduces the effects of misregistration. In the case of a gamma camera with a single scintillation detector, it is known to use a sliding window or "band" associated with the field of view of the scintillation detector to move in conjunction with the line source to aid in allowing the gamma camera to differentiate between detected transmission photons and emission photons. For example, reference is made to an article entitled, "A Scanning Line Source for Simultaneous Emission and Transmission Measurements in SPECT," by Patrick Tan, et al., published in the Volume 34, No 10, of the Journal of Nuclear Medicine, in October 1993. This reference discloses use of a single scanning line source with a single moving detection band.
However, this solution offered by Tan et al. does not adequately account for "side scatter" or "cross-talk" in emission studies involving two scintillation detectors (e.g., within dual head detector systems). Cross-talk in emission studies involves transmission photons scattering off of an object (e.g., cause by Compton scatter) being studied and improperly detected by a scintillation detector as emission photons. In a dual detector gamma camera, transmission photons emitted from a line source that is associated with a given detector may be improperly detected (e.g., after scatter) as proper emission photons by the other detector. This is the case because the scattered photon loses energy as a function of the scatter angle and changes energy level. Cross-talk may also occur from emission photons that scatter off the object. In dual head gamma cameras, the effects of cross-talk are dealt with by performing a post processing operation on the detected data. This post processing operation is time consuming and not entirely accurate. Therefore, it would be advantageous to eliminate the need for such post processing step by eliminating the detection of the cross-scattered photons.
The use of a single sliding detection window associated with a single gamma camera detector does little to prevent cross-talk in a dual detector system. What is needed is a system that is operable within a dual detector camera system that effectively eliminates the improper detection of emission photons due to cross-talk. The present invention offers such a system and solution.
In addition, there is a dual detector gamma camera system that employs tracking zoom regions (e.g., window regions) that are designed to track the motion of an object of interest during ECT motion. Within this system, the zoomed regions of the detector change as the detector rotates through (Emission Computed Tomography) motion about the object. Reference is made to U.S. Pat. No. 5,304,806, entitled, "Apparatus and Method for Automatic Tracking of a Zoomed Scan Area in a Medical Camera System," issued Apr. 19, 1994, and assigned to the assignee of the present invention, which discusses tracking zoom regions. The present invention provides for advantageous combination with the above system.
Accordingly, the present invention provides image data correction for nonuniform attenuation within a dual head camera system configurable between SPECT imaging and PET imaging modes of operation. It is another object of the present invention to provide such a system in a dual head system that eliminates cross-talk between the detectors during transmission and emission acquisition. It is another object of the present invention to provide such mechanisms and methods as described above that additionally allow effective use in conjunction with a tracking zoom region of a scintillation detector or tracking zoom regions of a pair of scintillation detectors. These and other objects of the invention not specifically recited above will become clear within discussions of the present invention herein.